JP-A-2011-113807 discloses a technology for detecting an abnormality in a power supply of a heating device and performing error processing. Specifically, in a case where the width of a zero-crossing signal is a predetermined time period or less, it is determined that a rectangular wave is being input to a fixing heater and supply of electric power to the fixing heater is interrupted.
Recently, due to electric power saving, after an AC power supply is turned off, a voltage of a power supply circuit tends to drop more gradually than before. In this case, even after the AC power supply is turned off, an operation time of a control device such as an ASIC for detecting an abnormality in a power supply of a heating device becomes longer. Therefore, there is a fear that a state where the AC power supply is in an OFF state and an AC voltage is not input may be erroneously determined as an abnormality in the power supply, and the error processing will be performed even in a case where the AC power supply is merely in the OFF state.